So a while back I got the EGR code and found out the stock Pre-cats were falling apart. I then replaced them with the Racingline High Flow Pre-Cats but I am now getting the P0420 Catalyst Efficiency Below Threshold code.

This is the list I got from the service manual for troubleshooting

- Three way catalyst converter Bank 2 -I just replaced them right?
- Engine Control Module (ECM) Program - Not sure what this is.
- Exhaust tube - Did have a leak but is now fixed and no flutter is heard at all. Would a leak that is so minor and is not audible still cause this code to go off?
- Intake air leaks - Don't hear anything leaking or see any damaged parts.
- Fuel injectors may be faulty - Not sure how to check
- Fuel injector leaks - Not sure how to check
- Spark plugs may be faulty - I am at 102K miles. Nissan recommends you change them at 105K miles. Could this be it?
- Improper ignition timing - Does this have to do with spark plugs going bad?
- Engine Control Module - Not sure what this is

Well the thing is the code keeps coming up I just keep erasing it. So its been coming up for about 2 months but I always thought it was the exhaust leak at the rear pre-cat. I mean I did fix it but all I did was tighten down the nuts that were loose. I didn't remove it all the way and apply new RTV gasket maker. I doubt that would be the problem though because everyone keeps telling me the RTV is just good to have as a back up measure and it should seal fine with out it.

You think the new pre-cat is faulty?

I would take it to Nissan but Im not under any kind of warranty so I know they would charge a ridiculous price and I'm sure they are going to blame it on the aftermarket pre-cats anyways.

I don't care if they blame stuff on my MODs, I know they are there. I just need them to diagnose it, so I can take it somewhere to get it fixed. How much is ridiculous in your mind?
$100 might be worth it. $500 Not at all.

So replace them both? I did. I think they go for about $115 each? Cheap fix if that's it and they really say their life is like 80k anyway. So it's almost like maintenance.

Exactly. I also think you should change your spark plugs. I don't care how long Nissan claims they are good for, I would never go 100k miles on a set of plugs. My Taurus plugs were supposed to be good for 100k miles...I changed them at 60k, and they needed it.

Exactly. I also think you should change your spark plugs. I don't care how long Nissan claims they are good for, I would never go 100k miles on a set of plugs. My Taurus plugs were supposed to be good for 100k miles...I changed them at 60k, and they needed it.

Yea I might even do that this weekend. I do need the upper intake manifold gasket from the dealer though right?

For the spark plugs I was planning on getting the NGK Laser Platinums since thats the oem replacement. Is there any advantage of getting the Iridium IX ones?

Also I wanted to run seafoam through my fuel system I just don't know if its worth it or not. I should do this right before I change my spark plugs right?

theres been a case on the other org and one member problem up here that the hfpc failed on them. the inside of them wore out and broke down. i would pull them off just to take a look that they are not clogged.

There are two O2 Sensors in the manifolds and one back in Y-pipe. The O2s in your Racingline HPC. Yeah about $115, I got some Bosch I believe from Napa or somewhere. Call around but don't skimp. Plugs, why not.

There are two O2 Sensors in the manifolds and one back in Y-pipe. The O2s in your Racingline HPC. Yeah about $115, I got some Bosch I believe from Napa or somewhere. Call around but don't skimp. Plugs, why not.

Seafoam, I think I would wait on that. One issue at a time.

I brought up the Seafoam because I know doing that fouls your spark plugs. I just didn't want to get these new spark plugs and foul them. I just don't know if it's really worth doing or not. I'm sure I would get more mpg right?

I'm not a seafoam guru, I really don't know anything about it, seems drastic to me but I am a seafoam newb. I run premium all the time and once in a while some BG cleaner or something like that. Never had issues with fouling.

You really need to change the plugs. If the code was present prior to doing the HFPC then you risk messing them up with something else wrong. Start with the plugs and go from there. Hold off on the seafoam. Get back to us once that is done.

I will start with replacing the spark plugs. I know I can get the NGK laser platinum's at Kragen I just have to make sure I can get that UIM gasket from Nissan and I should be able to do it either this weekend or the next.

theres been a case on the other org and one member problem up here that the hfpc failed on them. the inside of them wore out and broke down. i would pull them off just to take a look that they are not clogged.

Well for sure the front one is not damaged I did pull off that one when I fixed the exhaust leak. But the rear one I did not pull off because I assumed my problem were those loose nuts. I sure hope it's not the pre-cat falling apart.

Well for sure the front one is not damaged I did pull off that one when I fixed the exhaust leak. But the rear one I did not pull off because I assumed my problem were those loose nuts. I sure hope it's not the pre-cat falling apart.

usually you get that code for the rear cat. pull the cat for safe measures. atleast rule that out before investing in new o2 sensors. not too many threads on o2 sensors going bad.

Alright I was able to replace the spark plugs this last weekend. Honestly not very difficult just time consuming since there is so much attached to the UIM. I guess the next thing I can do is check on that rear pre-cat but I really don't want to because it's such a pain. I was thinking of maybe doing what Apollos suggested and look at it from the y pipe side. Maybe I can just remove the O2 sensor and look through that opening?